1. Field of the Invention
This invention relates to a rear face reflection mirror of multilayer film for synthetic resin optical part.
2. Related Art Statement
Recently, the frequency of using synthetic resin optical parts such as lens and the like becomes higher, and particularly there is a prism as a synthetic resin optical part, which is considered to be more demanded for use in camera, audiovisual eqipment or the like in future. The synthetic resin optical parts can attain the formation of complicated shapes and the reduction of cost and weight as compared with the optical parts made from glass. Particularly, they are easy in the working for the formation of prisms having different shapes or the like as compared with the glass optical parts.
In the formation of the prism, however, after the synthetic resin is worked into a prism by injection molding, cutting or the like, it is required to form a coating as a rear face mirror on an arbitrary slant surface of the prism. Heretofore, such a coating was formed by using the same evaporating material as in the glass optical part with a film structure.
In case of the synthetic resin optical parts, when the coating is carried out by using the same evaporating material with the same film structure as in the glass optical part, microcracks are generated after the film formation, or the adhesion property of the film to the substrate is insufficient, or even if there is no accident after the evaporation, microcracks are caused after the test for environmental resistance.
In general, when a pentaprism for camera or the like is used as a glass optical part, Ag is directly evaporated onto a glass substrate as a rear face mirror, and Cu is evaporated thereon as a protective film. On the other hand, when such a film structure is applied to a synthetic resin substrate such as acrylic resin or the like, the optical performances can be satisfied, but the durability performance can not be satisfied. That is, when the latter case is subjected to a peeling test with a cellulosic tape in which the cellulosic tape is adhered to the surface of the film and rapidly removed off therefrom, the film is easily peeled off from the synthetic resin substrate. Moreover, when a thermal shock test of a cycle {-20.degree. C..fwdarw. room temperature (20.about.25.degree. C.).fwdarw.60.degree. C.}is repeated 5 times as a test for environmental resistance, the occurrence of microcracks is observed. Even when Al is used instead of Ag and SiO is used as a protective film, the same phenomenon as described above is also caused, so that it is impossible to satisfy sufficient durability.